1. Field of the Invention
This invention relates generally to the field of gas turbine engines for the generation of electricity and, more particularly, to a method for gas turbine light-off.
2. Description of the Prior Art
The starting of a gas turbine engine is a complex operation. Typically, before the gas turbine engine can run on its own power, the engine must be accelerated by an external source, such as a battery, to provide sufficient airflow to the combustor for ignition, typically referred to in the industry as light-off. In a turbogenerator having a permanent magnet motor/generator coupled to a gas turbine engine, supplying electrical power to the permanent magnet motor/generator will function as a motor to drive the gas turbine engine. Typically, engine speed varies as a function of the torque versus speed characteristics of the starter motor.
The airflow to the combustor increases generally with gas turbine engine speed. Light-off occurs when the speed of the engine produces enough combustion air to produce the correct ratio of air with the fuel supplied. Because fuel flow is highly dependent upon ambient conditions, such as ambient temperature and atmospheric pressure, the amount of fuel flow to the gas turbine engine is actively controlled as a function of the speed of the gas turbine engine in order to achieve the correct fuel-to-air ratio for light-off.
Typically, in order for the correct fuel-to-air ratio to be achieved for light-off, atmospheric pressure and ambient temperature must be accurately known or, otherwise, the representation of fuel flow will not be accurate and will not result in a combustible fuel-to-air ratio. In addition, any deviation in the measurement of gas turbine engine speed, or in the correlation of combustion air with gas turbine engine speed, can easily cause difficulties in achieving the correct fuel-to-air ratio for light-off. Therefore, in a prior art starting procedure where speed (combustion air) and fuel flow are variable, light-off is not attempted until the correct fuel-to-air ratio is thought to have been achieved, generally through experience.
In another prior art starting procedure for gas turbine engine light-off, the gas turbine engine operates at a fixed speed to provide a substantially constant supply of combustion air for light-off, while the fuel flow is variable. The fuel flow is then ramped up to achieve the correct fuel-to-air ratio, at which point light-off occurs. This fixed speed method is insensitive to fuel control variations, gas turbine engine variations, and ambient conditions.
In any starting method for a gas turbine engine, after light-off occurs, the accelerator rate of the turbine engine increases rapidly thereby increasing the air flow. The exhaust gas temperature also increases rapidly indicating that light-off has occurred.
It is, therefore, an object of the present invention to provide a consistent method for light-off of a gas turbine engine wherein fuel flow is maintained at a substantially constant rate.
The present invention provides a method for light-off of a gas turbine engine that includes the following steps. First, ambient conditions and fuel heating value is determined. The ambient conditions can include ambient temperature and atmospheric pressure. Second, the gas turbine engine is accelerated at a preset acceleration rate to a speed to provide a supply of combustion air to a combustor for light-off of the gas turbine engine. The preset acceleration rate of the gas turbine engine can be determined based upon the ambient conditions and the fuel heating value. Acceleration of the gas turbine engine can be achieved by applying electrical assistance, such as battery power or electric grid, whereby a turbine drive shaft rotates causing compressor blades and turbine blades to rotate. Third, an ignition source, such as an igniter, is activated in the combustor of the gas turbine engine based upon the ambient conditions and the speed-of-rotation of the compressor blades and the turbine blades of the gas turbine engine. Fourth, a flow of fuel can be supplied through a fuel line to the combustor at a substantially constant rate to provide an optimum supply of fuel to the combustor. The supply of fuel at this constant rate is maintained until the correct fuel-to-air ratio is achieved and light-off of the gas turbine engine occurs. Fifth, light-off of the gas turbine engine can be sensed by using a heat sensor located adjacent the turbine exhaust gases to measure exhaust gas temperature of the gas turbine engine. Sixth, electrical assistance to the gas turbine engine is eliminated based upon a fixed value of the rotational speed of the turbine blades of the gas turbine engine. This fixed value can be determined by the torque versus speed characteristics of the starter motor. Seventh, the flow of fuel to the combustor is initiated through a primary fuel line and ramped up to increase the speed of the gas turbine engine to a final speed to which a load can be applied.
At least one fuel orifice which supplies fuel to the combustor is in fluid communication with a fuel source via the fuel line. The fuel line can include both a secondary fuel line and the primary fuel line. The fuel supplied to the combustor through the secondary fuel line is primarily used for light-off in the combustor. The secondary fuel line which supplies fuel to the combustor at a substantially constant rate can include a valve, such as a solenoid valve and/or a flow orifice to fix the flow of fuel to the combustor. However, the valve without the flow orifice in the secondary fuel line can also be used to fix the flow of fuel to the combustor by opening the valve to a predetermined position. The primary fuel line which supplies fuel to the combustor at a variable rate includes a primary fuel valve, such as a control valve. Fuel is primarily supplied through the primary fuel line after light-off has occurred. However, in a situation where the combustor is idled at an idle speed, the fuel supplied through the primary fuel line can be shut off, thereby only allowing fuel to be supplied through the secondary fuel line so that the combustor is operating at idle speed without any electrical assistance to the gas turbine engine.
In a situation where light-off does not occur after a fixed period of time at the constant flow rate of fuel, the system is then stopped and purged.
A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures wherein like reference characters identify like parts throughout.